Solar cells in general require two metal contacts to the semiconductor material, one of each polarity, to allow light generated charge from within the solar cell to be extracted and allowed to flow in external electrical wires as electricity. Most solar cells have one polarity of contact on the top surface and the opposite polarity metal contact on the rear surface. For example, in general, silicon solar cells have different metals for the front and rear contacts due to the different requirements when contacting n-type and p-type silicon. In addition, other attributes of the metal such as its electrical conductivity and thermal expansion coefficient as well as its cost affects, whether a metal is suitable and/or preferable as a metal contact on one or the other of the solar cell surfaces, must be considered. In general most silicon solar cells use different metals on a light receiving surface where shading losses, metal conductivity and contact resistance to the semiconductor are particularly important. On a non-light receiving surface where higher metal coverage and lower conductivity can be tolerated and where the polarity is opposite to the light receiving surface, other metals are usually preferable. For this reason, most screen-printed solar cells use high conductivity silver despite its high cost for the n-type front metal grid and cheaper aluminium to cover most of the p-type rear surface.
An alternative approach which may be used to apply metal contacts to a solar cell is via electroless plating, electro-plating with electrodes or light induced plating (LIP). One common problem with metal contacts formed to solar cell surfaces via these techniques is that the plating solutions have a high density of metal ions such that, when plating the metal nucleates at a particular site on the semiconductor surface, and that location then becomes the preferable site for continuing rapid plating which is fed from the high concentration of metal ions available in the solution. Unfortunately the already plated surface provides the most attractive site for further metal ion deposition making it difficult for plating to nucleate at other locations of the semiconductor surface. This leads to such locations plating upwards and outwards relatively quickly, with juxtaposed regions joining as the metal plates across the semiconductor surface rather than nucleating further growth from the surface. The result is relatively poor adhesion and contact resistance between the plated metal and the semiconductor surface.
Another common problem of prior. plating techniques is that they often lead to both polarities of silicon being plated with the same type of metal, rather than allowing the use of the most desirable metal for each contact. For example, with the LIP process, metal in electrical contact with the positive electrode of an illuminated solar cell can be transferred via a conductive liquid electrolyte to the n-type negative electrode where the metal is deposited/plated onto the exposed surface. This process is described in detail by Lawrence Durkee in U.S. Pat. No. 4,144,139 “Method of Plating by Means of Light”. As described by Durkee, one limitation of the LIP method is that it restricts the solar cell design to one that utilises the same metal for both polarities of electrodes. A second limitation is that it causes corrosion of the positive electrode metal towards the edges of the device due to its closer proximity to where the metal is to be deposited onto the negative electrode, which in turn leads to deterioration in the electrical conductivity of the metal contact to p-type material particularly towards the edges of the metal contact material. Both these limitations are unacceptable when fabricating high performance solar cells, with virtually all current commercial solar cells requiring different metals for the two polarities of metal contacts.
Another limitation of most LIP and electroless plating processes is that the growth rate tends to be conformal at best leading to the plating rate being more or less at a uniform rate in each direction. For many other applications this may be suitable, but for high efficiency solar cells, since shading of the top surface is roughly proportional to the width of the metal lines, it is desirable to have enhanced aspect ratios whereby the lines are as high as possible while being as narrow as possible. Conformal plating tends to result in the width of the metal lines increasing at a rate twice that at which the height increases, and results in outcomes such as that of the metal cross-section illustrated in FIG. 1 where the initial surface to be plated was 10 microns wide and which after conformal plating for about 10 minutes produced about 10 microns of plating in every direction, or equivalently a height of 10 microns but a width of about 30 microns.